In one of its aspects, the present invention relates to an improved, catalytic, solution process for preparing butyl rubber polymers. More particularly, the present invention relates to a process for preparing butyl rubber polymers with improved yields (catalyst efficiencies) at polymerization temperatures of xe2x88x92100xc2x0 C. to +50xc2x0 C. in low cost, inert, hydrocarbon solvents.
Conventional prior art processes for preparing butyl rubber polymers in solution (solution butyl processes) mainly employ aluminum trihalide catalyst systems, viz., those using aluminum trichloride, or aluminum tribromide alone (see, for example, U.S. Pat. Nos. 2,844,569 and 2,772,255). These prior art procedures are not wholly satisfactory, because they are performed at very low temperatures (e.g. xe2x88x9290xc2x0 C. to xe2x88x92110xc2x0 C.), leading to high refrigeration costs during polymerization. In addition, at such low temperatures polymer solutions have a very high viscosity and are difficult to handle. Further, a high viscosity of a polymer solution causes a very low rate of heat transfer, and also poor and difficult catalyst dispersion.
Aluminum trichloride has the disadvantage of having little or no solubility in many desirable hydrocarbon solvent systems, and is often introduced into the reaction feed as a solution in methyl chloride. Although aluminum tribromide is soluble in hydrocarbons, the use thereof can cause the undesirable formation of substantial amounts of very high molecular weight fractionsxe2x80x94see U.S. Pat. No. 2,772,255 (Ernst et al.).
Alkylaluminum dihalide catalysts are generally less reactive than the aluminum halides, but offer the advantage of excellent hydrocarbon solubility. To enhance their reactivity, they are frequently used together with cocatalysts.
Canadian patent 1,019,095 [Scherbakova et al. (Scherbakova)] teaches an industrial process for manufacturing butyl rubber in solution. The catalyst system in the process comprises an alkylaluminum halide (e.g. ethylaluminum sesquichloride ((C2H5)2AlCl.Cl2AlC2H5)), with water or hydrogen sulfide as a co-catalyst, and isopentane as a solvent. Not many details are known about the process, which most probably takes place at xe2x88x9285xc2x0 C. to xe2x88x9280xc2x0 C., with a content of solids in solution at about 10 weight percent.
Parker, et al. (U.S. Pat. No. 3,361,725) teaches that mixtures of dialkylaluminum halides (e.g., dialkylaluminum monochlorides) and monoalkylaluminum dihalides (e.g., monoalkylaluminum dichlorides), in which the latter component is present in small amounts, are effective solution butyl rubber catalysts, operate at far more economical (higher) temperatures and form excellent high molecular weight rubbers than then conventional methods. Usually, the rubber polymerizations using the above catalyst mixtures are conducted at temperatures ranging from about xe2x88x9287xc2x0 C. to xe2x88x9257xc2x0 C., and preferably at temperatures of xe2x88x9279xc2x0 C. to xe2x88x9268xc2x0 C., with excellent results being achieved with temperatures at or near xe2x88x9273xc2x0 C. at approximately atmospheric pressure.
In the Parker, et al. process, the polymers are soluble in the unreacted monomers, so that relatively minor amounts of diluent can be used. Reasonably small quantities of diluent can be employedxe2x80x94e.g., from 0 to 50 vol. percent diluent based on total volume of monomer and saturated catalyst solvent. Usually, however, the concentration of diluent during polymerization ranges from 0 to 20 vol. percent. Preferably, the C5 to C6 normal paraffins are usedxe2x80x94e.g., n-pentane and n-hexane.
The Parker, et al. catalyst mixture contains from about 2 to about 10 mole percent of the monoalkylaluminum dihalide and from about 90 to 98 mole percent of the dialkylaluminum monohalide. According to Parker, et al., this is the preferred way to achieve the most advantageous combination of ease of polymerization coupled with catalyst efficiency and good temperature control over the polymerization reaction. This latter characteristic is a significant advantage of the method. On the other hand, the reaction times require from about 50 to 100 minutes within the preferred temperature range.
In a previous invention [Canadian Patent Application 2,252,295 (Gronowski)], Bayer improved the Parker, et al. process by the direct addition of minute amounts of water or aluminoxane to the catalyst solution. The use of this improved catalytic system resulted in higher reaction rates and the formation of a rubber having a higher molecular weight than that formed using the Parker, et al. catalyst. This made it possible to carry out polymerizations at even more economical (i.e. higher) temperatures than with the method taught by Parker, et al., with the resulting rubber still displaying desirable properties. In addition, good temperature control was maintained during polymerizations, while the isoprene content of the rubber was higher than that of control reactions using the conventional Parker, et al. catalyst.
Dialkylaluminum halides alone do not catalyze butyl rubber polymerization. Monoalkylalumium dihalides can initiate the polymerizations on their own, but, as mentioned before, to enhance their reactivity they are frequently used together with cocatalysts. To date, such cocatalysts have been organometallic halides or halogen-containing organic compounds of one sort or another.
It would be useful to have a method for the manufacture of butyl rubber having better yields (catalyst efficiencies) than those attainable to date using an alkylaluminum dihalide and which does not have a negative impact on the molecular weight of the resulting polymer.
It has been determined that alkylaluminum dihalides can be activated by the direct addition of a small amount of aluminoxane, and that an efficient catalyst system based on a mixture of these two compounds can be used for the manufacture of butyl rubber.
It is an object of the present invention to provide a novel method for the manufacture of butyl rubber using a catalyst system based on alkylaluminum dihalides activated by a direct addition of a small amount of aluminoxane.
Accordingly, the present invention provides a process for preparing a butyl polymer with improved catalyst efficiencies based on alkylaluminum dihalide as a catalyst, the process comprising contacting a C4 to C8 monoolefin monomer with a C4 to C14 multiolefin monomer at a temperature in the range of from about xe2x88x92100xc2x0 C. to about +50xc2x0 C. in the presence of a diluent and a catalyst mixture comprising a monoalkylaluminum dihalide and an aluminoxane.